1. Field of the Invention
The present invention generally relates to drop-out detecting apparatus and, more particularly, is directed to a drop-out detecting apparatus which can accurately detect a drop-out from a high frequency video signal reproduced from a record medium even when the level of the high frequency video signal is fluctuated or even when the level thereof is extremely small.
2. Description of the Prior Art
In the most simplified drop-out detecting apparatus for detecting a drop-out from a high frequency video signal (frequency-modulated video signal) played back from a magnetic tape by a rotary magnetic head in a video tape recorder (VTR), the reproduced high frequency video signal is envelope-detected and the resultant envelope signal is compared with a constant reference level, whereby the portion of the compared signal less than the reference level is generated as a drop-out detecting signal.
The above conventional drop-out detecting apparatus has a drawback such that, if the reference level is not changed in accordance with the change of the level of the reproduced high frequency video signal, then it cannot detect the drop-out accurately.
In order to remedy the above-mentioned drawback, another conventional drop-out detecting apparatus is proposed, in which the reproduced high frequency video signal whose gain is automatically controlled is envelope-detected, the resultant envelope signal is compared with the constant reference level and the portion of the compared signal less than the reference level is generated as the drop-out detecting signal.
The above conventional drop-out detecting apparatus becomes complicated in arrangement because it needs an automatic gain control (AGC) circuit and also has the drawback such that it cannot detect a drop-out if the level of the high frequency video signal is fluctuated during the time when the rotary magnetic head comes in contact with the magnetic tape.
Other drop-out detecting apparatus is further proposed to remedy the above shortcoming. According to this previously-proposed drop-out detecting apparatus, as shown in FIG. 1, the reproduced high frequency video signal is envelope-detected and the resultant envelope signal is attenuated by a predetermined attenuation ratio of, for example, -16 dB or -20 dB, thereby produced as a reference level. Then, the envelope signal is level-compared with the reference level and the portion of the signal lower than the reference level is generated as a drop-out detecting signal.
According to the above drop-out detecting apparatus in which the reproduced high frequency video signal is envelope-detected, the resultant envelope signal is attenuated by the predetermined attenuation ratio so as to be generated as the reference level, the envelope signal is compared with the reference level and the portion of the signal lower than the reference level is produced as the drop-out detecting signal, the drop-out can be accurately detected because the reference level changes in proportion to the level of the envelope signal of the reproduced high frequency video signal. The above drop-out detecting apparatus, however, cannot produce the drop-out detecting signal if a signal-to-noise (S/N) ratio of an FM-demodulated output of the high frequency video signal is deteriorated in the signal portion, the level of which is made extremely small, such as when the rotary magnetic head scans a guard band area of a slant track formed on a magnetic tape if the level of the high frequency video signal, i.e., envelope signal is changed comparatively gently and is considerably fluctuated as in the still playback mode as shown in FIG. 2.